With the application of thermal flying height (FH) control (TFC) technology, the Wallace equation based in-situ FH testing technology has become the main approach to measure the FH of read/write (R/W) heads. In general, one single harmonic of a readback signal is enough for detecting variations in the FH. However, in actual applications, the read head may go off-track and such off-track of the read head changes the amplitude of the readback signal obtained from the storage medium and may be mis-counted as a change in the FH. This is one of the major sources contributing to the FH testing error.